Varieties resistant to root-knot nematodes have been the first line of defense against the root pest.

However, a disturbing new trend in resistance breakdown has been noted in some of California's processing tomato growing regions.

It has also been reported in other countries such as Greece, Spain, France, and Morocco, according to Antoon Ploeg, University of California, Riverside entomologist.

The reasons for this resistance breakdown are not entirely clear. It could be a factor of environmental conditions such as temperature, changes in the nematode, changes in the resistant plants or something else. It could be a combination of factors, according to Ploeg. “Even if a plant is resistant, that doesn't mean it is completely resistant or won't be damaged.”

One of the problems is that all resistant tomato varieties have the same resistant gene — the Mi-gene — that came from a single cross between a wild tomato plant and a commercial tomato plant made back in the early 1940s.

“The more you plant a resistant variety, the more a nematode is likely to change and infest your resistant variety,” Ploeg says. “It doesn't matter if you're growing resistant variety A, B or C because they all have the same resistant gene.”

Although resistant varieties can still aid in combating nematode infestations, researchers say that an integrated management approach should be adopted to boost the effectiveness of resistant varieties.

“Temperature plays an important role in the life cycle of nematodes. They become active when the soil temperature is above about 64 degrees F and their optimum temperature is about 90 degrees F.”

At the lower range of the active temperature scale, root-knot nematode can complete a life cycle in seven weeks. As temperatures warm to 85-90 degrees F that time frame can be reduced to only three weeks.

Root-knot nematodes enter plant roots where they reproduce and impede the uptake of water and nutrients. If a grower plants resistant varieties, root-knot nematodes still enter the plant roots. However, they cannot reproduce.

“It's a very nice system to control nematodes,” Ploeg says. “However, it only works if the soil temperatures are below 82 degrees F at the time the root-knot nematode enters the plant root.”

Additionally, resistant tomato varieties are only resistant to three types of root-knot nematodes — Meloidogyne incognita, m. javanica, and m. arenaria. “Fortunately, those are the three species that commonly occur in Central and Southern California,” Ploeg says.

Ploeg added, if resistance is breaking down, reducing the frequency of resistant tomato varieties in a rotation is a good idea.

“If you continually use resistant varieties, the chances of developing an aggressive nematode are much higher,” Ploeg says.

Additionally, reducing initial populations of root-knot nematodes through either chemical means or crop rotation can help reduce pressure on the management system.

Finally, there is a desperate need for plant breeders to find and introduce other types of resistant genes other than the Mi-gene into tomato varieties, according to Ploeg.

“We've already seen resistance breaking in nematode populations in the Sacramento Valley,” says Brenna Aegerter, San Joaquin County UC farm advisor. “I suspect it is only a matter of time before we see it in the rest of the San Joaquin Valley as well.”